Rotary hearth furnace for use in the iron and steel industry

ABSTRACT

A rotary hearth furnace for use in the iron and steel industry comprises a furnace ( 12, 112 ) with plan in the shape of an annulus, closed at the bottom by a rotary hearth ( 14, 114 ), lined at the top with refractory material ( 15, 115 ), and a base ( 28, 128; 30, 130 ) of the furnace ( 12, 112 ). Said hearth ( 14,114 ) comprises a plurality of sectors of an annulus ( 17, 117; 17′, 117′ ), all the same as one another and connected to form an annulus, complementary to that of the internal plan of the furnace ( 12, 112 ), which rotate around the central axis of the annulus, by means of two concentric sets of wheels ( 26, 126 ) arranged according to two circumferences, set at equal instances, with supports ( 25,125 ), fixed to the base ( 28 ) or below the hearth ( 114 ), complementary to two circular rails ( 20, 120 ), fixed respectively below the hearth ( 14 ) or the base ( 128 ). According to the invention, both said sets of wheels ( 26, 126 ) and said two rails ( 20, 120 ) are positioned in such a way as to have an equal load distribution.

The present invention refers to a rotary hearth furnace for use in theiron and steel industry.

Rotary hearth furnaces have been used for a long time, particularly inthe iron and steel sector.

Their uses are very varied. For example they are used to heat metals iningots, slabs or blooms, before rolling; or for the heat treatment ofmaterials, such as metal parts, glass or graphite; or again forprocessing loose or agglomerated raw materials such as coal or alumina,or mixtures of raw materials such as iron ore with carbon materials, orwaste rich in iron with carbon materials.

Rotary hearth furnaces are built in different shapes and with a diametervarying from a few metres to more than 50 m, with width even larger than6 m.

The hearth rotation speeds are also variable. Large heating furnacesrotate at even less than one revolution per hour, while small rotaryfurnaces, for calcination or for processing raw materials, reach forexample fifteen revolutions per hour.

The hearth is generally in the shape of an annulus and rotates throughtwo circumferential sets of wheels. These are located on thecircumferences close to the ends of the annulus.

The wheels run on rails, and two different construction solutions arepossible.

A first solution is to make wheels integral with the frame of the rotaryhearth and rails fixed to the ground, mounted on very rigid structures,often made of reinforced concrete.

The second solution is to make rails integral with the frame of therotary hearth and wheels fixed to the ground, mounted on very rigidstructures, often made of reinforced concrete.

In the latter case the rotary hearth must be planned and builtconsidering a fatigue stress in the metal structure, and consequently inits refractory lining, due to the continuous changing of the points ofcontact between the wheels and the rails during rotation of the hearth.

This fatigue stress may be very critical for the life of the refractoryand so the furnaces are planned with wheels positioned on the twodiameters, internal and external, on the same radii, so as to be able todivide the above metal structure into sectors having the same angularspacing as the wheels. In doing this, the deflection of the hearthstructure due to the changing of the position of the point of contact ofthe wheels with the rails applied on the structure generates limitedstresses and eliminates or minimises the cyclical movements of therefractory.

New processes have recently been developed in the field of the treatmentof iron ore which require rotary hearth furnaces with very large hearthareas, and in some cases with very high rotating speeds, even more than15 revolutions per hour.

These rotary hearth furnaces require hearth surfaces larger than thosebuilt up till now, with diameters even larger than 50 m and hearthwidths larger than 6 m, even over 10 m.

In these furnaces certain problems, which are not important in thetraditional applications, become critical when the dimensions and therotating speeds are increased so considerably. The main problems to betackled are the wear of the coupling between wheels and rails, and thecurving of the hearth panels due to the difference in temperature in thepanel supporting structure.

Normally the wheels and the rails are generally positioned on twocircumferences very close to those of the ends of the hearth. Due to thegeometry of the system, the wheels on the outer surface are more loadedthan the wheels on the internal circumference. With the increase in thewidth of the hearth, this load difference is increased and consequentlythere may be great differences in the wear of the wheels and of therails, which are on the two internal and external circumferences. Thebehaviour described above may be compensated, for example, by changingthe size of the wheels and of the rails.

In these furnaces, also the planarity of the hearth is of primaryimportance. As is known, the supporting beams of the refractory hearthare subject to heating due to heat conduction through the hearth and atthe same time they are cooled by irradiation and conduction with theenvironment below. This normally generates, in these supporting beams, adifference in temperature between the top and the bottom of the hearth,giving rise to a phenomenon of curving of the hearth when it reaches theworking temperature.

When the width of the hearth increases, the traditional constructionwith frames having panels as wide as the hearth itself becomes criticalon account of the strains due to said thermal effects and the consequentstresses or strains which may be generated in the refractory structure.

Moreover, since some new rotary hearth furnaces need to be installed ata considerable height above ground level, even higher than 20 metres,the high supports of the furnace, which must support the wheels or therails of the hearth, increase the flexibility of the structure. Inparticular, if the structure, under the load of the hearth, generatesdifferent deflections from point to point, and particularly in anasymmetric manner, the induced stresses may be very critical, provokingfunctional complications and phenomena of fatigue during operation ofthe furnaces.

The general aim of the present invention is to improve, in a rotaryhearth furnace, the performances of the wheels and of the rails, and todecrease the stresses on the structures of the rotary hearth and of itsrefractory lining.

Another aim is to overcome the above-mentioned existing inconveniencesof the conventional construction technique in an extremely simple,economic and particularly functional manner.

In view of the above aims, according to the present invention, it hasbeen intended to realise a rotary hearth furnace for use in the iron andsteel industry, having the characteristics stated in the enclosedclaims.

The structural and functional characteristics of the present inventionand its advantages with respect to the prior art will be even more clearand evident from an examination of the following description, referringto the enclosed drawings, which show a rotary hearth furnace for use inthe iron and steel industry realised according to the innovativeprinciples of the invention.

In the drawings:

FIG. 1 shows a layout view from below of only a metal bearing structureof a rotary hearth for a furnace, according to a first realisation of afurnace of the present invention;

FIG. 2 is a section taken according to the plane II-II of FIG. 1, thatis according to a radial plane, of a first realisation of a rotaryhearth furnace for use in the iron and steel industry, the rotary hearthof which is shown in FIG. 1;

FIG. 3 is a side elevation section enlarged and developed on the plane,partly showing means for rotating the hearth according to therealisation in FIG. 2, with wheels placed on a fixed structure;

FIG. 4 is a side elevation section, made according to a radial plane ofhalf the furnace, of a second realisation of a rotary hearth furnace foruse in the iron and steel industry;

FIG. 5 is a side elevation section enlarged and developed on the plane,partly showing means for rotating the hearth according to therealisation in FIG. 4, with wheels placed on the mobile structure of thefurnace.

With reference above all to FIGS. 1, 2 and 3, a rotary hearth furnacefor use in the iron and steel industry according to the invention, in afirst possible example of an embodiment, is indicated overall as furnace12, placed on a support structure 16 and equipped with a rotating hearth14.

The furnace 12 has a plan in the shape of an annulus lined withrefractory material and it is closed at the side and at the top by walls13 lined on the inside with refractory material. Instead, at the bottomthe furnace 12 is closed by the hearth 14, also in the shape of anannulus but rotating around a central vertical axis of the annulus. Thishearth 14 is lined at the top with refractory material 15, for examplewith refractory panels.

The hearth 14 is composed of a series of annulus sectors 17. As well asthis circumferential division of the sectors 17, there may be, as shownin FIG. 1, a radial division of the annulus of the hearth 14, breakingthe sectors 17 into two semi-sectors 17′ along arcs 18 of anintermediate circumference between the two end circumferences, internaland external, of the annulus.

The intermediate circumference which comprises the arcs 18 is such as todivide the sector 17 into two semi-sectors 17′ of the same weight. Dueto geometric considerations on the areas subtended by semi-sectors of anannulus, it is therefore larger than the median circumference of theannulus of the hearth 14.

The division into semi-sectors 17′ of the hearth allows a considerabledecrease of the hearth level variations due to the thermal curving of asupport structure of the sectors 17 when the hearth reaches the normalworking temperatures.

The semi-sectors 17′ are shown above a reticular structure comprisingcross members 22, uprights 23 and 23′, and possibly tie rods orstiffening struts 24.

As may be seen in FIGS. 2 and 3, the reticular structure has two annularbars 19 at the bottom, concentric with the annulus of the hearth 14.These bars 19 are placed on circumferences coinciding with the centre ofgravity of the sectors 17 and 17′, so that the weight of these sectorsis discharged directly on the system below.

Below these bars 19 there are two rails 20, having the same section.

In the example in FIG. 2 these bars 19 are placed in a position suchthat the weight of the hearth 14, bearing down on the two bars 19 isalmost identical.

The support structure 16 comprises a base 28, placed on circumferentialsets of columns 30. Fixed on this base 28 are two sets of supports 25for wheels 26, placed along a circumference, so that the wheels 26 arecomplementary to and operatively aligned with the two rails 20 of thehearth 14.

The two sets of wheels 26 are placed in such a way that the pairs ofwheels 26 are positioned on the same radii of two concentriccircumferences, internal and external, these radii being spaced at equaldistances on the same circumferences. The number of these pairs ofwheels 26 is equal to those of the sectors 17. In this way the stressesdue to the changing of position, during rotation of the hearth 14itself, of the forces applied by the wheels 26 on the bars 19 andtherefore on the sectors 17 and on the refractory material 15, areminimised. In fact the semi-sectors 17 and 17′ are principally supportedby the uprights 23 which join them vertically to one of the two annularbars 19. These uprights 23, as may be seen in FIG. 2, are in factlocated in an area close to the centre of gravity of the semi-sectors17′.

Each sector 17 is thus equipped with two uprights 23, one for eachsemi-sector 17′, connected to the two bars 19. The two bars 19 areconnected to the cross members 22, placed in a radial position.

The cross members 22 may be placed corresponding to the two uprights 23of each sector 17. An upright 23′ is added to the two uprights 23, inthe area of the separating arc 18 between the two semi-sectors 17′,having the aim of absorbing any vertical force that could be generatedif the position of the uprights 23 were not exactly in the centre ofgravity of said semi-sectors 17, 17′ and also to give stability to thesemi-sectors 17, 17′ themselves.

It should be noted that the best condition is the one in which theuprights 23 are in the centre of gravity of the semi-sectors 17′. Inthis case the vertical stresses in the arc 18 that divides the twosemi-sectors 17′ are cancelled and the division may be used as a thermalexpansion joint. Moreover, the fact that the cross member 22 is notstressed by the loads transmitted to the hearth corresponding to theupright 23′ allows the avoidance of possible phenomena of deflection ofthe beam, thus optimising the work of the wheels 26 on the rails 20.

In the FIG. 2 are shown circumferential sets of columns 30 (in this casefour) in such a way that the columns 30 are aligned in groups on thesame radii of concentric circumferences, where these radii are spaced atequal distances on the same circumferences. The number of these groupsof columns 30 is equal to that of the sectors 17.

More particularly, with reference to FIGS. 2 and 3, two of the fourcircumferences, on which the columns 30 are placed, may be equal to thecircumferences on which are placed the supports 25 for the wheels 26,and these supports 25 are located on the base 28 corresponding to eachcolumn 30.

As the columns 30 are designed in such a way as to have all the samevertical deflection under the load of the hearth 14, the stresses on thehearth 14, due to changing of the points of application of the load ofthe bars 19 on the wheels 26 during rotation, are thus minimised.

Instead, if the columns 30 cannot be positioned as described, thesupport structure 16 must be designed in such a way that the verticaldeflection of the wheels 26 is as identical as possible.

FIGS. 4 and 5 illustrate a further possible practical embodiment of theinvention, where the components equal to and/or equivalent to thoseillustrated in FIGS. 1, 2 and 3 are marked with the same referencenumbers, increased by 100.

This second embodiment differs from the first only in the fact that thereciprocal position between the supports 25 of the wheels 26 and therails 20 indicated in the first embodiment is inverted.

As may be seen in FIGS. 4 and 5, in this embodiment the rails 120 arefixed to the base 128.

Instead the supports 125 are anchored below the annular bars 119. Moreprecisely, the supports 125 are paired and positioned on the same radiiof the two concentric circumferences of the bars 119, spaced at equaldistances on the circumferences themselves. The number of these pairs ofsupports 125 is equal to that of the sectors 117. Moreover the supports125 are fixed corresponding to the uprights 123 of the reticularstructure that holds up the hearth 114.

In so far as regards the support structure 116, the precaution is alwaystaken to position the circumferential sets of columns 130 in such a waythat they are aligned, in groups, on the same radii of the concentriccircumferences, where the radii are spaced at equal distances on thecircumferences themselves. The number of these sets of columns 130 isequal to that of the sectors 117.

In this way the wheels 126 of the hearth 114, spaced at equal distancesin the same way as the columns 130, do not exert any differential stresson the hearth 114 when this is being rotated. The hearth is in factsubject to a uniform lifting and lowering movement due to the deflectionof the rails 120 and of the underlying support structure 116. It doesnot produce stress on the structure of the hearth 114 and does notinduce movements in the refractory material 115 placed above it.

Finally, a rotary hearth furnace for use in the iron and steel industryaccording to the invention, where the structure that supports the hearthhas a median radius equal to the one by which the hearth is divided intotwo concentric annuli having the same load, stresses the wheels of thetwo internal and eternal circumferences in an identical or very similarmanner.

In this way the behaviour of the wheels is the same and the constructioncan be simplified by using wheels and rails of the same size.

In addition to simplifying the construction, and therefore also themaintenance, there are other advantages, such as an improvement inaccessibility to the furnace seals.

Moreover, with maximum benefit when the width of the furnace isconsiderable, it is possible to divide the hearth structure not onlyradially into sectors, but also by splitting it along the circumferenceinto semi-sectors; this is facilitated by positioning the wheels almoston the centre of gravity with respect to the two semi-sectors and thusminimising stresses in the area where the sectors are divided intosemi-sectors and torsional stress on the annular bars.

These semi-sectors generate various benefits. Above all, the torsionaleffect on the support structures of the rails or wheels mounted on thehearth is eliminated or minimised. Then there is a reduction of themaximum radial heat expansion on the sectors of the structure becausethis is now divided into two semi-sectors, anchored in the centre.Lastly, dividing the sectors into semi-sectors, the total bending valueof the hearth sectors, due to the differences in temperature between thetop and the bottom of the metal structures which make up said sectors,is reduced with respect to the solution without semi-sectors.

From the above description with reference to the figures, it appearsevident that a rotary hearth furnace for use in the iron and steelindustry with large dimensions according to the invention isparticularly useful and advantageous. The aims mentioned in theintroduction to the description are thus achieved.

The forms of the rotary hearth furnace for use in the iron and steelindustry according to the invention can of course be different from theone shown purely as an example without limitation in the drawings, justas the materials may be different.

The area of protection of the invention is therefore defined by theenclosed claims.

1) Rotary hearth furnace for use in the iron and steel industry,comprising a furnace (12, 112) with plan in the shape of an annulus,closed at the bottom by a rotary hearth (14, 114), lined at the top withrefractory material (15, 115), and a base (28, 128; 30, 130) of thefurnace (12, 112), wherein said hearth (14, 114) comprises a pluralityof sectors of an annulus (17, 117; 17′, 117′), all the same as oneanother and connected to form an annulus, complementary to that of theinternal plan of the furnace (12, 112), which rotate around the centralaxis of the annulus, by means of two concentric sets of wheels (26, 126)arranged according to two circumferences, set at equal distances, withsupports (25, 125), fixed to the base (28) or below the hearth (114),complementary to two circular rails (20, 120), fixed respectively belowthe hearth (14) or the base (128), characterised in that both said setsof wheels (26, 126) and said two rails (20, 120) are positioned in sucha way as to have an equal load distribution. 2) Rotary hearth furnaceaccording to claim 1, characterised in that both said sets of wheels(26, 126) and said two rails (20, 120) are positioned symmetrically withrespect to a circumference which divides the annulus of the hearth (14,114) into two concentric annuli loaded in the same way, saidcircumference being larger than the median circumference of said annulusof the hearth (14, 114). 3) Rotary hearth furnace according to claim 1,characterised in that both said wheels (26, 126) of the two sets are thesame as one another, and that said rails (20, 120) have the samesection. 4) Rotary hearth furnace according to claim 1, characterised inthat both said wheels (26, 126) are aligned, two by two, on the sameradii of the two concentric circumferences of the two sets of wheels(26, 126), and are of the same number as the sectors (17, 117). 5)Rotary hearth furnace according to claim 1 or 4, characterised in thatsaid sectors (17, 117) are divided into two semi-sectors (17′, 117′)along arcs (18) of an intermediate circumference between the two endcircumferences, internal and external, of the annulus of the hearth (14,114), said arches (18) dividing the sectors (17, 117) into two equallyloaded semi-sectors (17′, 117′). 6) Rotary hearth furnace according toclaim 5, characterised in that said semi-sectors (17′, 117′) areconnected to the rails (20), or, respectively, to the supports (125) ofthe wheels (126) by means of vertical uprights (23, 123), positionedcorresponding or close to their centre of gravity. 7) Rotary hearthfurnace according to claim 1, characterised in that said base (28, 128)is held up by a support structure (16, 116), comprising concentriccircumferential sets of columns (30, 130), aligned in groups on the sameradii of said circumferences, where said radii are spaced at equaldistances from one another, and having the same number of sectors (17,17′). 8) Rotary hearth furnace according to claim 7, characterised inthat said groups of columns (30, 130) are of the same number as thesectors (17, 117) of the hearth (14, 114), said columns (30, 130) havingthe same deflection under the load of the hearth (14, 114). 9) Rotaryhearth furnace according to claim 8, characterised in that said sectors(17, 117) are divided into two semi-sectors (17′, 117′) along arcs (18)of an intermediate circumference between the two end circumferences,internal and external, of the annulus of the hearth (14, 114), wheresaid arcs (18) divide the sectors (17, 117) into two equally loadedsemi-sectors (17′, 117′), said semi-sectors (17′, 117′) being connectedto the rails (20), or, respectively, to the supports (125) of the wheels(126) by means of vertical uprights (23, 123), positioned correspondingor close to their centre of gravity. 10) Rotary hearth furnace accordingto claim 1, characterised in that the furnace (12, 112) has a plan inthe shape of an annulus with large dimensions.